1. Field of the Invention
The present invention relates to a receiving apparatus and a receiving method for receiving a radio frequency (RF) signal such as, for example, a terrestrial digital broadcasting wave and the like in mobile terminals, vehicle-mounted terminals and so on while moving or at stationary sites, an LSI (Large Scale Integration) for a radio frequency signal, and an LSI for a base band signal. More particularly, the present invention relates to an AGC independent control technique for selecting a channel of an RF signal received by an antenna, and performing an automatic gain control (AGC) for RF amplification and intermediate frequency (IF) amplification in an RF unit independently. This application claims priority under 35 U.S.C. §119 to Japanese Patent Application Serial No. 2007-167508 filed Jun. 26, 2007, the entire disclosure of which is hereby incorporated by reference herein.
2. Description of the Related Art
Japanese Patent Application Publication No. 2002-290178 discloses a technique for AGC in a receiving apparatus, the RF receiving apparatus having a frequency conversion circuit that converts an RF signal received by an antenna into an IF signal, including an RF variable gain amplifier (hereinafter amplifier being abbreviated as “Amp”) that amplifies the RF signal; a mixer that amplifies a particular channel from an output signal of the RF gain Amp and outputs the IF signal; first and second IF variable gain Amps that amplify the IF signal output from the mixer; a demodulation circuit that demodulates the IF signal from the second IF variable gain Amp; a detection circuit that detects a level of the IF signal from the second IF variable gain Amp and controls a gain of the second IF variable gain Amp based on a result of the detection; and a comparison circuit that compares an output of the detection circuit with a preset level and controls gains of the RF variable gain Amp and the first IF variable gain Amp based on a result of the comparison. The RF variable gain Amp generally includes a low noise Amp (LNA) that amplifies an RF signal with low noise.
The disclosed RF receiving apparatus performs gain control of the second IF variable gain Amp until the detection level of the received input signal reaches a predetermined value, and performs gain control of the RF variable gain Amp when the detection level exceeds the predetermined value. In this manner, in performing an AGC operation to keep the level of the input signal for the demodulation circuit constant, it is possible to perform the AGC operation at high speed and to deal with a strong input.
Japanese Patent Application Publication No. 2005-102008 also discloses a technique for AGC in a receiving apparatus, wherein the receiving apparatus and receiving method are capable of improving reliability of AGC by changing a gain of a RF variable gain Amp and keeping a gain of an IF variable gain Amp constant when a received signal is large, and by keeping the gain of the RF variable gain Amp constant and changing the gain of the IF variable gain Amp when the received signal is small.
However, the receiving methods and apparatuses as noted above have the following problems.
A first problem is considered with reference to FIG. 5 of the present application. FIG. 5 is a waveform diagram showing examples of an RF variable gain Amp gain control (RF-AGC) operation and an IF variable gain Amp gain control (IF-AGC) operation in a conventional receiving apparatus. The conventional receiving apparatus changes a gain of one of the RF variable gain Amp and the IF variable gain Amp depending on input levels IN1 to IN3, which are input levels of a signal received by an antenna. For example, the gains of the RF variable gain Amp and the IF variable gain Amp are enlarged for small input levels up to input level IN1, and as the input level increases to IN2, the gain of the IF variable gain Amp is decreased to the minimum level and the gain of the RF variable gain Amp remains unchanged. As the input level further increases to input level IN3 corresponding to the maximum value of dynamic range, the gain of the RF variable gain Amp decreases to the minimum level. However, in such a gain changing method, since an RF gain variable range is different from an IF gain variable range, it is not possible to perform proper gain control due to a difference between RF response speed and IF response speed.
A second problem is considered with reference to FIG. 6 of the present application. FIG. 6 is a schematic waveform diagram of frequency vs. signal level (intensity), showing interference between a desired received signal and adjacent channel signals. In the prior art, when a plurality of interference with very strong power from adjacent channels are received out-of-band from the desired received signal, a band pass filter (BPF) may be used at input sides of an LNA and a mixer to filter out undesired signals. However, in the absence of a filter that limits a frequency band to a desired signal band, undesired signals may occur due to saturated amplification, thereby deteriorating the received signal and line quality. Accordingly, there arises a need to control an RF variable gain Amp constituted by an LNA so that it does not have excessive gain. However, there also arises a need for the RF variable gain Amp to have sufficient gain in order not to deteriorate signal quality such as a noise figure (NF) of the LNA. There is a need to achieve optimal gain control from these conflicting constraints.
However, in the conventional receiving method, as shown in FIG. 6, when the power of the interference is large compared to the small power of the desired received signal, the gain of the RE variable gain Amp becomes maximal, which results in a higher possibility of saturated amplification due to signal interference. For example and as shown in FIG. 6, in some cases a signal level (intensity) of a desired received signal CH0 may become decreased, and signal level (intensity) of adjacent channel signals CH1 and CH2 may become enlarged under a phasing effect, so that a synthetic signal is produced due to interference at a receiving point between a direct wave and a reflected wave due to fast movement of a mobile receiver. In this case, when the signals CH0, CH1 and CH2 are amplified, the desired signal CH0 interferes with the amplified adjacent channel signals CH1 and CH2, which results in deterioration of reception quality of the desired signal CH0.